Method and apparatus for unloading a rotary compressor

ABSTRACT

A method and apparatus are disclosed for relieving the fluid pressure at the outlet of an air compressor when the latter pressure exceeds the pressure of air in a compression cell of the compressor arriving at the outlet. The air pressure at the outlet is relieved by blocking backflow of air under pressure at a point downstream from the outlet and transferring air under pressure at the outlet to a point downstream of the point of backflow blocking. Blocking is achieved by means of a fluid pressure responsive check valve in a flow path between the outlet and a point of use for the compressed air, and transfer of air under pressure from between the outlet and the check valve is achieved by an auxiliary compressor, having a smaller input capacity than the output of the main compressor. During operation of the main compressor to deliver its rated volume of air at its rated pressure the auxiliary compressor and check valve cooperate to deliver the compressor output toward the point of use.

United States Patent 11 1 Caffrey Dec. 11, 1973 METHOD AND APPARATUS FOR UNLOADING A ROTARY COMPRESSOR [75] inventor: Terrence Cafirey, Kent, Ohio [73] Assignee: The Davey Compressor, Kent, Ohio [22] Filed: Apr. 7, 1972 21 Appl. No.: 242,166

[51] Int. Cl. F04b 23/04 [58] Field of Search 417/286, 62, 251, 417/426, 295

[56] References Cited UNITED STATES PATENTS 1,753,280 4/1930 Baumann et a1 417/251 X 2,218,565 10/1940 Vickers 417/62 1,814,857 7/1931 Rosle 417/62 X 1,049,894 l/l9l3 Merrill 417/62 2,599,701 6/1952 Eames 417/62 2,761,387 9/1956 Gaubatz 417/62 3,168,236 2/1965 Lambe rton et a1. 417/295 FOREIGN PATENTS OR APPLICATIONS 114,967 3/1925 Switzerland 417/62 Primary Examiner-Carlton R. Croyle Assistant Examiner-Richard Sher Attorney-James H. Tilberry [57] ABSTRACT A method and apparatus are disclosed for relieving the fluid pressure at the outlet of an air compressor when the latter pressure exceeds the pressure of air in a compression cell of the compressor arriving at the outlet. The air pressure at the outlet is relieved by blocking backfiow of air under pressure at a point downstream from the outlet and transferring air under pressure at the outlet to a point downstream of the point of backflow blocking. Blocking is achieved by means of a fluid pressure responsive check valve in a flow path between the outlet and a point of use for the compressed air, and transfer of air under pressure from between the outlet and the check valve is achieved by an auxiliary compressor, having a smaller input capacity than the output of the main compressor. During operation of the main compressor to deliver its rated volume of air at its rated pressure the auxiliary compressor and check valve cooperate to deliver the compressor output toward the point of use.

12 Claims, 1 Drawing Figure METHOD AND APPARATUS FOR UNLOADING A ROTARY COMPRESSOR This invention relates to the art of fluid compressors and, more particularly, to the unloading of fluid pressure at the outlet of a compressor to reduce the load on the compressor drive.

The present invention relates ,to fluid compressors, such as air compressors, which are driven directly or indirectly by a suitable motor or internal combustion engine to compress a fluid and deliver the compressed fluid to a point of use such as might be defined, for example, by pneumatic tools such as jackhammers. While the invention will be described in conjunction with a particular type of air compressor, it will be understood that the principles of the invention are applicable to other types of compressors and to the compressing of fluids other than air.

Compressors of the above character include a housing having a chamber therein and a driven component or components in the chamber which are driven by the drive motor. The driven member or members and chamber cooperate to define one or more compression cells adapted to receive air through a variable inlet leading to the chamber and to compress the air to a cell pressure for discharge through an outlet from the chamber, wherebytair at a discharge pressure is available for delivery to a point of use. Such compressors have a rated intake capacity, compression ratio and discharge pressure. The speed of the compressor drive motor and the sizeof the air inlet opening are generally controlled in accordance with discharge pressure so that compressor operation is achieved in accordance with the demand for compressed air at the point of use and so as to prevent the compressor output pressure from exceeding the rated magnitude. Accordingly, if the discharge pressure is at the rated magnitude and there is less than full demand for air delivery, the speed of the compressordrive is below the maximum therefor and the air inlet opening is throttled. When the dis-,

charge pressure decreases, such as upon a demand for more compressedair at the point of use, the speed of the compressor drive increases and air intake throttling decreases.

-When the compressor is operating at its rated intake capacity and its rated pressure the compressor drive is operating near or at a maximum speed therefor and air intake throttling is at a minimum, whereby the compression cells of the compressor are filled with a maximum volume of airuWhen the discharge pressure exceeds the ratedrnagnitude, the speed of the compressor drive is reduced and air intake is throttled as mentioned above. Throttlingof the air intake reduces the pressure "of air in the compression cells of the compressor,

whereby the fluid pressure in a given cell at the time of discharge of the air in the cell to the compressor outlet is less than the discharge pressureat the compressor outlet. Thus, there is a backflow of air through the compressor outlet into the cells caused by the pressure differential, and this back pressure imposes a load on upon communication of the cells and compressor outlet.,Upon the latter circumstances the compressor, is considered to be operating unloaded. When the compressor is operating at its rated capacity and pressure, such as when there is a full demand for compressed fluid at the point of use, the compressor is considered to be operating fully loaded. It has been found by tests that the load on a compressor drive motor during unloaded compressor operation is about 60 to percent of the load on the drive motor when the compressor is operating fully loaded. Any excess load on the compressor drive is, of course, disadvantageous both economically and with respect to the life of the drive means.

It is accordingly an outstanding object of the present invention to provide for decreasing the discharge pressure at the outlet of a compressor when the discharge pressure exceeds that in the compressor cells moving into communication with the compressor outlet.

Another object of the present invention is to provide for reducing the discharge pressure at the outlet of a compressor by blocking backflow through the compressor outlet and transferring fluid from between the compressor outlet and the point of backflow blocking.

Yet another object of the present invention is to provide for decreasing the discharge pressure at the outlet of a compressor in a manner whereby backflow of fluid at discharge pressure into the compressor is reduced to advantageously decrease the load otherwiseimposed on the compressor drive.

Still another object of the present invention is to reduce the discharge pressure at the outlet of a compressor by providing a second compressor of smaller capacity in series with the outlet of the compressor and in parallel with backflow preventing means for the second compressor to transfer fluid between the outlet and backflow preventing means to a point downstream of the backflow preventing means. i

Yet a further object of the present invention is the provision of a method to reduce the load imposed on the drive means of a fluid compressor during periodsof compressor operation in which a fluid back pressure is exerted on the compressor through the discharge outlet thereof. 1

The foregoing objects, and others, will in part be obvious and in part more fully pointed out hereinafter in conjunction with the description of the isometric drawing of a preferred embodiment of the present invention.

Referring now to the drawing in detail wherein the showings are for the purpose of illustrating a preferred embodiment of the presentinvention only and not for the purpose of limiting the same, a compressor 10 is illustrated which includes a housing 12 having a cylindrical chamber 14 therein and in which a rotary, bladed type compressing member 16 is disposed. Rotor 16 is suitably mounted on shaft 18 having one end thereof extending outwardly of one end of housing 12 for connection with compressor drive means 20 such as might be defined, for example, by an internal combustion engine having a drive shaft directly or indirectly. coupled to shaft 18 to impart rotation thereto. It will be appreciated that many drive arrangements may be employed and that the drive means can be defined by motor means other than an internal combustion engine.

Rotor 16 is provided with a plurality of radially slidable blades 22, and rotor 16 and shaft 18 are eccentrically mounted in chamber 14 relative to the chamber axis by suitable bearing means at opposite ends of the rotor. The space between circumferentially adjacent blades 22, the outer surface 24 of the rotor therebetween and the inner surface of chamber 14 defines a compression cell for fluid to be compressed. Chamber 14 is provided with an inlet 26 and an outlet 28 at circumferentially spaced locations and, in a manner well known, fluid to be compressed enters a compression cell through inlet 26 and is carried by the cell and compressed therein during rotation of the rotor toward outlet 28. When the cell moves into communication with outlet 28 the compressed fluid flows through the outlet thus to provide fluid at the outlet at a discharge pressure. Fluid under pressure leaving the outlet is of course adapted to be delivered to a point of use, as described more fully hereinafter.

In addition to inlet opening 26, the fluid inlet portion of compressor includes control means 30 which is operable to vary the amount of air entering the compression cells. In this respect, control means 30 is a fluid actuated variable valve assembly including a housing 31 having a fluid intake opening 32 and a fluid discharge opening 34 leading to inlet opening 26. A reciprocable valve element 36 is supported within housing 31 and is actuated by diaphragm component 38 to vary the quantity of air flowing through intake opening 32 toward outlet opening 34. Diaphragm 38 is biased by spring 40 in a direction to increase the size of the opening of intake 32 and is displaced by fluid under pressure entering chamber 42 therebehind to move valve element 36 in the direction to close opening 32. In the embodiment illustrated, fluid under pressure for actuating diaphragm 38 is air delivered to chamber 42 through line 44 which leads to chamber 42 from the discharge side of the compressor. Means such as a suitable pressure responsive valve in line 44, is provided to prevent delivering of air to chamber 42 until the discharge air pressure reaches a predetermined magnitude such as, for example, the rated discharge pressure. It will be appreciated therefore that when air is discharged at or above the rated pressure valve element 36 is moved in a direction to close intake opening 32. Similarly, a reduction in discharge pressure from the rated magnitude results in movement of valve element 36 in the opposite direction by spring 40 to increase the airinlet opening. In addition to the air inlet control described above, air at outlet or discharge pressure from the compressor is delivered to drive motor 20 through a line 46 leading thereto from the discharge side of the compressor. Air flowing through line 46 is operable in conjunction with suitable control means associated with the motor to control the speed of the drive motor in accordance with compressor outlet pressure. In this respect, an increase in discharge pressure is operable to reduce engine speed and a reduction in discharge pressure is operable to increase the speed of the drive means. The speed control for the drive means and the air intake control are operable together to provide for a more uniform compressor operation in response to varying demands for compressed air during compressor operation. These controls are well known and do not form a part of the present invention and, accordingly, a more detailed description thereof is not believed necessary.

Preferably, oil is introduced into the compressor during its operation in a well known manner to provide cooling, lubricating and sealing functions. For this purpose, an oil separator unit 48 is employed in conjunction with the compressor and includes an oil reservoir section 50 from which oil 52 is delivered through line 54 to an oil filter assembly 56 having an outlet passage 58 through which oil is delivered to chamber 14. Suitable oil cooler means 59 is provided, and the oil delivered from reservoir 52 is circulated through the oil cooler before delivery to chamber 14 if the oil is above a predetermined temperature. As described more fully hereinafter, compressed air discharged from the compressor is delivered through separator 48 to a point of use, and the separator operates to remove oil entrained in the compressed air. In this respect, the separator includes filtering means 60 through which the compressed air passes for flow through separator outlet 62 leading to the point of use. Filter means 60 has an oil collecting pan or the like 64 associated therewith and in which oil separated from the compressed air collects. An aspirator line 66 has one end thereof disposed in the oil in pan 64 and the other end thereof is connected to fluid inlet housing 28, whereby oil is drawn from pan 64 and delivered to the fluid inlet housing by inlet air flow and thence through discharge opening 34 and chamber opening 26 of a compressor.

As mentioned hereinabove, the compressed air discharged from compressor 10 is adapted to be delivered to a point of use through separator 48 upon demand for compressed air at the point of use. In the embodiment illustrated, the point of use is illustrated schematically at 68. The manner in which the compressed air is used at this point is not important to the present invention. For purposes of the present description, however, it will be assumed that an operator controlled pneumatic tool is located at the point of use and is connected to the outlet of separator 48 by suitable conduit means illustrated schematically and which may be defined, for example, by a flexible hose. In any event, when air under pressure is required at the point of use to operate the pneumatic tool the operator actuates a control valve to permit air under pressure from the compressor to enter and operate the tool. Upon actuation of the tool in this manner, air under pressure is delivered to the tool causing a drop in discharge pressure at the outlet of the compressor. In response to the discharge pressure drop the compressor drive and air inlet con-' trols are actuated in a manner whereby the compressor is operated to increase its output and discharge pressure toward the rated values thereof. During operation of the compressor it will be driven at varying speeds depending on the demand for compressed air at the point of use and the discharge pressure at the compressor outlet.

There will be occasions during compressor operation when air at a given discharge pressure flowing through lines 44 and 46 to the air intake control and drive motor control will operate to move air intake valve 36 in the closing direction to reduce the amount of air entering the compressor and to reduce the speed of the drive means to decrease the speed of compressor rotor 16. Such a condition may exist when, for example, the operator closes the valve of the pneumatic tool at the point of use 68 whereby there is no demand for compressed air, or when for some other reason the discharge pressure exceeds the rated discharge pressure. In either event, the resulting decrease in the amount of intake air delivered to the compression cells results in a decrease in the pressure of the air in the cells and thus the pressure of the air delivered to compressor outlet 28. Thus, as the cells move into communication with outlet 28 there is a backflow of fluid at the high discharge pressure into the cells imposing an undesirable load on the drive motor for the rotor. The largest load imposed on the compressor drive motor in this manner results when the intake valve 36 is substantially or completely closed and there is no demand for compressed air at the point of use. Under these circumstances, when a given compression cell communicates with chamber inlet opening 26 a vacuum is drawn in the cell and the air pressure at outlet 28 is at or above the rated magnitude, whereby a substantial pressure differential exists between the cell and the compressor outlet when the cell moves into communication with the outlet. Therefore, there is a substantial backflow of air under pressure through the compressor outlet to the compression cell resulting in a considerable load on the drive motor for the compressor. While this load will be less than the load imposed on the drive motor when the compressor is operating to deliver compressed air at its rated pressure to the point of use, it has been found that the load imposed by backflow under these circumstances is from between 60 to 70 percent of the full load. It becomes desirable then to decrease the air pressure at the compressor outletwhen the latter pressure exceeds the pressure of air delivered tothe compressor outlet in order to reduce the load on thedrive motorv ln accordancewith the present invention, the discharge pressure of the compressor is reduced under the foregoing circumstances and in response to the occurrence thereof by blocking backflow to the compressor outlet and transferring air under pressure from the area between the compressor outlet and the point of backflow blocking. In the embodiment illustrated in the drawing, the output of compressor is directed toward the point of use 68 along a flow path which includes separator 48 and line 70. The flow path further includes a flow path portion 72 between compressor outlet 28 and separator 48. Flow path portion 72 includes a secondcompressor 74 of smaller input capacitythan the output of compressor 10, and a fluid flow control valve 76. The second compressor and valve are connected in fluid flow parallel relationship and have the inlet sides thereof in flow communication with compressor outlet 28and the outlet sides thereof in flow communication with separator 48. More particularly, a flow line 78 extends from compressor outlet 28 to separator 48and valve 76 is interposed in line 78 downstream from outlet 28. Compressor 74 has an inlet 80 connected to line 78 by means of line 82 and has an outlet 84 connected to separator 48 through line 86.

Compressor 74 is disposed in a housing 88 defining a chamber 90 in which a rotor 92 is disposed. Rotor 92 is similar to rotor 16 and in this respect includes a plurality of radially slidable blade components 94 cooperable with the rotor and chamber to define compression cells. Rotor 92 is mounted on a shaft 96 supported in housing 88 for rotation about an axis eccentric to the axis of chamber90. Compressor 74 may be supported relative to compressor 10 in any suitable manner and in the embodiment illustrated is mounted on one end of compressor 10 forshaft 96 to be axially aligned with shaft 18 of compressor 10. Preferably, compressor 74 is adapted to bedriven by drive motor of compressor 10 and, accordingly, the adjacent ends of shafts 18 and 96 are suitably interconnected by coupling means 98. Any suitable coupling means may be employed and may, for example, rigidly interconnect shafts 18 and 96 for the shafts to be rotatable at the samespeed at all times. The coupling means may, however, advantageously be in the form of a clutch operable in response to suitable control means 100 to provide for selectively disengaging compressor 74 from compressor 19. Moreover, it will be appreciated that the drive of compressor 74 by drive motor 20 could otherwise be achieved such as through suitable belt, chain or gear train assemblies, and that compressor 74 can be operated at a different speed from that of compressor 19, as pointed out more fully hereinafter. Still further, it will be appreciated that an integral common shaft could be employed for compressor rotors 16 and 92 rather than coupled shaft elements. v

Fluid flow control valve 76, in the embodiment illustrated, includes a ball valve element 102 biased by a coil spring 104 toward engagement with a valve seat 106 provided in valve housing 108. Thus, it will be appreciated that the valve permits fluid flow in the direction from outlet 28 of compressor 10 toward separator 48 and prevents backflow in the opposite direction by engagement of ball valve 102 with seat 106. The bias of spring 104 need only be of a magnitude sufficient to seat ball 102 because valve 76 is not intended to be responsive to a particular magnitude of fluid pressure exerted against the upstream side thereof in order to open. Accordingly, it will be appreciated that valve 76 could be vertically disposed and that ball 102 could be biased into engagement with seat 106 by its own weight only. Further, it will be appreciated that other forms of pressure responsive valves can be employed. Line 86 from compressor 74 opens into valve housing 108 downstream of ball valve 102, but it will be appreciated that line 86 could open into line 78 downstream of valve 76 or could extend to and open directly into separator 48.

As mentioned hereinabove, second or auxiliary compressor 74 has an input capacity less than the output of compressor 10. Thus, during normal operation of compressor 10 to deliver air below or at itsrated outlet pressure, the'second compressor is not operable to reduce the outlet pressure thereof. Auxiliary compressor 74 can be of any" size smaller thanthat-compressor l0, and an auxiliary compressor havingan intake capacity V4 the output of compressor 10 has been found to work quite satisfactorily. As an illustrative example only, a main compressor having a 100 cfm input capacity, an 8:1 compression ratio and a rated pressure of 100 psi will have an output, fully loaded, of 12.5 cfm at rated pressure. Therefore, an auxiliary compressor having an input capacity of about 3.1 cfm would be desirable. Further, the primary purpose of auxiliary compressor 74 is to deliver or transfer the air under pressure between outlet 28 and valve 76 downstream of the valve to prevent backflow into compressor 10 as described hereinabove. Accordingly, it is advantageous to reduce or eliminate any unnecessary work in connection with operation of the auxiliary compressor during normal operation of main compressor 10 when there is no backflow problem. This can be achieved as mentioned above by providing for the auxiliary compressor to be decoupled from drive motor 10 or otherwise rendered inoperable during normal operation of compressor 10. Another approach, particularly suitable when the auxiliary compressor is directly coupled to drive motor 20 or is otherwise operated to be driven conti uously with compressor 10, is to provide for the auxiliary compressor to have a very low compression ratio of from about 1:1 to 2:] and preferably from about 1:1 to 15:1. Any work or loading of the drive motor 20 or a separate drive means for the auxiliary compressor resulting from driving the auxiliary compressor or during periods of normal operation of compressor 10 is superfluous and is advantageously reduced when the auxiliary compressor is continuously operated by providing for the auxiliary compressor to have a low compression ratio. If auxiliary compressor 74 is operated independently of compressor l and only during those periods when scavanging of compressed air at discharge pressure from the outlet of compressor, 10 is desired, then it becomes advantageous to provide for the auxiliary compressor to have a higher compression ratio than 2:1 to further decrease the load on the drive motor 20 by providing less exposed compressor blade area at the discharge port of the auxiliary compressor against which back pressure exerts a force to load the drive.

The operation of compressor 74 and valve 76 to reduce the discharge pressure of compressor 10 will be described assuming compressor 74 to have a compressed air input capacity A the output of compressor 10 and to be coupled therewith for rotors l6 and 92 to be driven simultaneously at the same rotational speed. Upon start-up of compressor 10 by drive motor 20, rotors l6 and 92 are driven simultaneously and the compressed air output of compressor 10 is delivered to separator 48 by compressor 74 until the output of compressor 10 reaches the input capacity of compressor 74 as measured, for example, in cubic feet per minute. When the output of compressor 10 reaches the input capacity of compressor 74 the output pressure of compressor 10 opens valve 76 and the remainder of the output of compressor 10 flows to separator 48 through valve 76. It will be appreciated, therefore, that when compressor 10 is operating to deliver its rated capacity, 25 percent of the output thereof is transferred from one side of valve 76 to the other by compressor 74 which, accordingly, merely operates to transfer'compressed air across the valve and does not operate to reduce the discharge pressure at the outlet of compressor 10.

Presuming that compressor 10' is operating at its rated capacity and air intake valve 36 moves in the direction to close inlet 32 in response,for example, to a decrease in demand for compressed air at point of use 68, less air is drawn into compressor 10 resulting in a lower pressure being established in the compression cells thereof when the cells move into communication with outlet 28. Since the pressure of air in the cells arriving at outlet 28 is less than the discharge pressure existing at the outlet, air under pressure at the outlet tends to flow back therethrough and into the compression cells. Such backflow is blocked by ball valve 102 which closes against seat 106 because the downstream pressure on the ball valve now exceeds the upstream pressure thereon. At the same time, the compressed air between compressor outlet 28 and valve 76 which is substantially at the discharge pressure of compressor 10, is transferred to the downstream side of valve 76 by second compressor 74, whereby the discharge pressure at outlet 28 of compressor 10 is reduced to reduce the work load on drive motor 20. Thus, compressor 74 operates to scavange air under pressure between outlet 28 and valve 76. It will be noted that as the two compressors continue to operate under these conditions a back pressure load similar to that removed from compressor 10 exists at the discharge of compressor 74 and that this load is imposed as a work load on drive motor 20 since the drive motor is common to both compressors. The work load thus imposed on the motor by compressor 74, however, is much less than that which would be imposed thereon by compressor 10 if the discharge pressure of compressor 10 was not reduced.

As mentioned hereinabove, the work load on motor 20 in response to unloaded operation of compressor 10 would be 60 to percent of the work load on the motor when the compressor is operated fully loaded. If the input of compressor 74 is /4 the output of compressor 10 and compressor 74 has a compression ratio of 1:1, only about 25 percent of the power otherwise required to drive compressor 10 unloaded is required when the auxiliary compressor is employed. Thus, the work load imposed on drive motor 20 during unloaded operation of compressor 10 is, in the embodiment described, only 25 percent of the 60 to 70 percent of full work load heretofore imposed on drive motor 20. In other words, the work load imposed on drive motor 20 during unloaded operation of compressor 10 in the present embodiment, is only 15 to 17.5 percent of the full work load requirement imposed on drive motor 20 in operating compressor 19 at its rated capacity. It will be appreciated, therefore, that the present arrangement provides for a substantial decrease in the requirements of power from drive motor 20 during operation of compressor 10 in an unloaded condition. Further, the 15 to 17.5 percent work load can be advantageously reduced by employing an auxiliary compressor having a compression ratio greater than 1:1 so that less blade area is exposed at the outlet thereof. Accordingly, if the auxiliary compressor is operated only when pressure unloading is desired and does not have to be driven continuously by the drive motor it becomes advantageous to use an auxiliary compressor having a higher compression ratio.

The work load imposed on drive motor 20 during operation of compressor 10 from a fully loaded toward an unloaded condition will, of course, vary upwardly from the minimum percent work load imposed when compressor 10 is operating fully unloaded but, in any event, will be less than the work load which would be imposed on the drive motor in the absence of the discharge pressure reducing arrangement of the present invention. It will be further appreciated, that the percent work load achieved as described above will vary in accordance with the ratio of size between compressor 10 and second compressor 74. For example, if compressor 74 were of a size or capacity one-half the output of compressor 10 and had a compression ratio of 1:1, the power requirements of drive motor 20 during fully unloaded operation of compressor 10 would be 30 to 35 percent of the power required to drive compresv sor 10 fully unloaded without the discharge pressure reduction arrangement of the present invention. As mentioned above, this percentage would be reduced by use of an auxiliary compressor having a compression ratio greater than lzl. In order for the discharge pressure reduction arrangement of the present invention to be operable, it is only necessary that the second compressor have a smaller input capacity than the output of the first compressor. The ratio of capacities of compressors l0 and 74, therefore, may be any desired ratio. The four-to-one ratio described hereinabove is merely one ratio which has been found to be highly satisfactory in providing the desired results.

If, in the embodiment described hereinabove, it is desired to operate the second compressor 74 only during periods in which the main compressor 10 is fully unloaded, control means 100 can be actuated to cause declutching or uncoupling of the second compressor from the drive means during other periods of main compressor operation. If such decoupling is employed, the total compressed air output of compressor 10 would then be delivered to the point of use through valve 76. Further, the clutch or coupling means could be controlled so as to cause compressor 74 to be actuated in response to the existence of the condition to be avoided, namely the existence of a discharge pressure at outlet 28 exceeding the pressure existing in the compression cells upon communication thereof with outlet 28. Accordingly, control means 100 could, for example, be a pressure responsive control operable in response to a sensed pressure delivered thereto from a suitable point in the compressor system.

While considerable emphasis has been placed herein on the fact that the two compressors are radially sliding vane type compressors, it will be appreciated that the present invention is readily applicable to other types of compressors. For example, the two compressors could be screw-type compressors. Moreover, the two compressors need not be of the same type. In this respect, one of the compressors could be a screw type compressor and the other a radially sliding vane type compressor. Still further, itwill be appreciated that the two compressors could be physically separated and the drive shafts thereof interconnected with one another or separately interconnected with the drive means. Still further, the two compressors may have different rotational speed requirements foroperation at the rated capacity output thereof, whereby the shafts thereof would be suitably interconnected with one another or with drive means in a manner whereby each would be operable atits own rated speed. Still further, it will be appreciated that the two compressors could be independently driven by corresponding drive means. In the latter arrangement, a smaller drive unit would be required to drive the second compressor and, while the latter drive would be loaded during operation of the main compressor fully unloaded, the load on the larger drive means required for the main compressor would still advantageously be reduced during operation of the main compressor fully unloaded.

Since many possible embodiments ofthe present invention may be made and since many possible changes may be madein the embodiment herein described, it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the present invention and not as a limitation.

I claim:

1. An air compressor arrangement including housing means having inlet and outlet openings, driven rotor means in said housing and cooperable therewith to provide compression cell means for receiving air through said inlet compressing the air and delivering the compressed air to said outlet, means for controlling the volume of air entering said cell means through said inlet and thus the pressure of air delivered to said outlet by said-cell means, said control means including means responsive to the pressure of air discharged through said outlet to reduce the volume of air entering said cell means when said discharge pressure exceeds a predetermined magnitude, whereby said cell means then delivers compressed air to said outlet at a pressure below said predetermined magnitude, flow path means for compressed air discharged through said outlet, and means in said flow path to reduce the discharge pressure at said compressor outlet when said discharge pressure exceeds said predetermined magnitude and said control means reduces the volume of air entering said cell means, said pressure reducing means including means to block backflow of air under pressure toward said outlet and means to transfer air under pressure from between said outlet and blocking means to a point in said flow path downstream from said blocking means.

2. The compressor arrangement according to claim 1, wherein said blocking means is tluid flow responsive valve means operable to permit air flow along said path in the direction leading from said outlet.

3. The compressor arrangement according to claim 1, wherein said transferring means is a second compressor having second driven rotor means, and means interconnecting said driven rotor means of said compressor and said second driven rotor means of said second compressor.

4. The compressor arrangement'according to claim 3, wherein said second compressor has an input capacity less than the output capacity of the first named compressor.

5. The compressor arrangement according to claim 4, wherein said input capacity is about one-quarter of said output capacity.

6. The compressor arrangement according to claim 5, wherein said second compressor has a compression ratio of from about 1:1 to 2:1.

7. The compressor arrangement according to claim 3, wherein said blocking means is fluid pressure responsive valve means operable to permit air flow along said path in the direction leading from said compressor outlet.

8. The compressor arrangement according to claim 7, including shaft means supporting said rotor means and second rotor means for rotation, drive means, and means interconnecting said shaft means and drive means for said drive means to rotate said shaft means.

9. The compressor arrangement according to claim 7, wherein said second compressor and fluid pressure responsive valve means have corresponding inlet and outlet ends, said inlet ends being in flow communication with said compressor outlet and said outlet ends being in flow communication with one another, whereby said second compressor and valve means are in parallel fluid flow relationship with respect to one another. L

10. The compressor arrangement according to claim 9, wherein said second compressor has an input capacity less than the output capacity of the first named compressor.

11. The compressor arrangement according to claim 10, wherein said input capacity is about one-quarter of said output capacity.

12. The compressor arrangement according to claim 11, wherein said second compressor has a compression ratio of from about 1:1 to 2:1. 

1. An air compressor arrangement including housing means having inlet and outlet openings, driven rotor means in said housing and cooperable therewith to provide compression cell means for receiving air through said inlet compressing the air and delivering the compressed air to said outlet, means for controlling the volume of air entering said cell means through said inlet and thus the pressure of air delivered to said outlet by said cell means, said control means including means responsive to the pressure of air discharged through said outlet to reduce the volume of air entering said cell means when said discharge pressure exceeds a predetermined magnitude, whereby said cell means then delivers compressed air to said outlet at a pressure below said predetermined magnitude, flow path means for compressed air discharged through said outlet, and means in said flow path to reduce the discharge pressure at said compressor outlet when said discharge pressure exceeds said predetermined magnitude and said control means reduces the volume of air entering said cell means, said pressure reducing means including means to block backflow of air under pressure toward said outlet and means to transfer air under pressure from between said outlet and blocking means to a point in said flow path downstream from said blocking means.
 2. The compressor arrangement according to claim 1, wherein said blocking means is fluid flow responsive valve means operable to permit air flow along said path in the direction leading from said outlet.
 3. The compressor arrangement according to claim 1, wherein said transferring means is a second compressor having second driven rotor means, and means interconnecting said driven rotor means of said compressor and said second driven rotor means of said second compressor.
 4. The compressor arrangement according to claim 3, wherein said second compressor has an input capacity less than the output capacity of the first named compressor.
 5. The compressor arrangement according to claim 4, wherein said input capacity is about one-quarter of said output capacity.
 6. The compressor arrangement according to claim 5, wherein said second compressor has a compression ratio of from about 1:1 to 2:
 7. The compressor arrangement according to claim 3, wherein said blocking means is fluid pressure responsive valve means operable to permit air flow along said path in the direction leading from said compressor outlet.
 8. The compressor arrangement according to claim 7, including shaft means supporting said rotor means and second rotor means for rotation, drive means, and means interconnecting said shaft means and drive means for said drive means to rotate said shaft means.
 9. The compressor arrangement according to claim 7, wherein said second compressor and fluid pressure responsive valve means have corresponding inlet and outlet ends, said inlet ends being in flow communication with said compressor outlet and said outlet ends being in flow communication with one another, whereby said second compressor and valve means are in parallEl fluid flow relationship with respect to one another.
 10. The compressor arrangement according to claim 9, wherein said second compressor has an input capacity less than the output capacity of the first named compressor.
 11. The compressor arrangement according to claim 10, wherein said input capacity is about one-quarter of said output capacity.
 12. The compressor arrangement according to claim 11, wherein said second compressor has a compression ratio of from about 1:1 to 2:1. 